Single/Dual/Quad, Micropower,

Single-Supply Rail-to-Rail Op Amps

10

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MAX495

10k

1

5

NULL

4

NULL

Figure 2. Offset Null Circuit

__________Applications Information

The dual MAX492, quad MAX494, and single MAX495

op amps combine excellent DC accuracy with rail-to-

rail operation at both input and output. With their preci-

sion performance, wide dynamic range at low supply

voltages, and very low supply current, these op amps

are ideal for battery-operated equipment and other low-

voltage applications.

Rail-to-Rail Inputs and Outputs

The MAX492/MAX494/MAX495’s input common-mode

range extends 0.25V beyond the positive and negative

supply rails, with excellent common-mode rejection.

Beyond the specified common-mode range, the out-

puts are guaranteed not to undergo phase reversal or

latchup. Therefore, the MAX492/MAX494/MAX495 can

be used in applications with common-mode signals at

or even beyond the supplies, without the problems

associated with typical op amps.

The MAX492/MAX494/MAX495’s output voltage swings

to within 50mV of the supplies with a 100k

Ω load. This

rail-to-rail swing at the input and output substantially

increases the dynamic range, especially in low supply-

voltage applications. Figure 1 shows the input and out-

put waveforms for the MAX492, configured as a

unity-gain noninverting buffer operating from a single

centered at +1.5V. The output amplitude is approxi-

Input Offset Voltage

Rail-to-rail common-mode swing at the input is obtained

by two complementary input stages in parallel, which

feed a folded cascaded stage. The PNP stage is active

for input voltages close to the negative rail, and the

NPN stage is active for input voltages close to the posi-

tive rail.

The offsets of the two pairs are trimmed; however, there

is some small residual mismatch between them. This

mismatch results in a two-level input offset characteris-

tic, with a transition region between the levels occurring

at a common-mode voltage of approximately 1.3V.

Unlike other rail-to-rail op amps, the transition region

has been widened to approximately 600mV in order to

minimize the slight degradation in CMRR caused by

this mismatch.

To adjust the MAX495’s input offset voltage (500µV max

at +25°C), connect a 10k

Ω trim potentiometer between

the two NULL pins (pins 1 and 5), with the wiper con-

circuit is ±6mV. External offset adjustment is not avail-

able for the dual MAX492 or quad MAX494.

The input bias currents of the MAX492/MAX494/MAX495

are typically less than 50nA. The bias current flows into

the device when the NPN input stage is active, and it

flows out when the PNP input stage is active. To reduce

the offset error caused by input bias current flowing

through external source resistances, match the effec-

tive resistance seen at each input. Connect resistor R3

between the noninverting input and ground when using

Figure 1. Rail-to-Rail Input and Output (Voltage Follower

Circuit, VCC = +3V, VEE = 0V)